To eliminate the need for mounting a constructional machine physi cally accurately on the turning center axis and easily mount the machine in the vicinity of the turning center where a mounting position is restricted much, by detecting a turning angle by a combination of a magnet and a magnetic sensor.

A magnetic sensor 9 is mounted in the upper part of a center joint spindle 4a. A magnet 10 is fitted to a supporting member 12 fixed to a center joint housing 4b so that its N pole is directed in the turning center axis Z. A magnet 11 is fitted to a supporting member 13 fixed to the center joint housing 4b so that its S pole is directed in the turning central axis Z. A pair of signals from the sensor 9 are input to an operation circuit consisting of a CPU and a peripheral circuit. The operation circuit operates a relative turning angle of a turning body 2 to a running body 1, based on the signals.